Device for detecting the degree of deterioration of a catalyst

ABSTRACT

A catalyst deterioration detecting device comprising a first air-fuel ratio sensor and a second air-fuel ratio sensor which are arranged in the exhaust passage upstream and downstream of the catalyst, respectively. When the air-fuel ratio of the mixture fed into the engine is changed over from the lean air-fuel ratio (A/F) L  to the rich air-fuel ratio (a/F) R , the air-fuel ratio detected by the second air-fuel ratio sensor is changed to the rich air-fuel ratio (A/F) R  after it is maintained at the stoichiometric air-fuel ratio for a time AT R . The deterioration of the catalyst is detected based on the time T R , the amount of air and the difference between the rich air-fuel ratio (A/F) R  and the stoichiometric air-fuel ratio.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a device for detecting the degree ofdeterioration of a catalyst.

2. Description of the Related Art

In an engine, a catalyst is normally arranged in the exhaust passage topurify the exhaust gas. Such a catalyst, for example, a three waycatalyst has an O₂ storage function such that it absorbs and storesexcess oxygen existing in the exhaust gas when the air-fuel ratio of theair-fuel mixture fed into the engine cylinder becomes larger than thestoichiometric air-fuel ratio, i.e., when the air-fuel mixture becomeslean, and that the catalyst releases oxygen when the air-fuel ratio ofthe air-fuel mixture fed into the engine cylinder becomes smaller thanthe stoichiometric air-fuel ratio, i.e., when the air-fuel mixturebecomes rich. Accordingly, where the air-fuel ratio is alternatelychanged on the rich side and the lean side of the stoichiometricair-fuel ratio, since excess oxygen is absorbed and stored in the threeway catalyst due to the O₂ storage function thereof when the air-fuelmixture becomes lean, NO_(x) is reduced. Conversely, when the air-fuelmixture becomes rich, since the oxygen which has been absorbed andstored in the three way catalyst is released therefrom, HC and CO areoxidized. Accordingly, Nox, HC and CO can be purified at the same time.

Therefore, in a conventional engine, an air-fuel ratio detector fordetecting the air-fuel ratio of the air-fuel mixture fed into the enginecylinder is arranged in the exhaust passage upstream of the catalyst.When the air-fuel mixture becomes lean, the amount of fuel to be fed isincreased and, when the air-fuel mixture becomes rich, the amount offuel to be fed is reduced. As a result, the air-fuel ratio isalternately changed on the rich side and the lean side of thestoichiometric air-fuel ratio, and thus the amount of NOx, HC and CO isreduced at the same time.

However, if the three way catalyst deteriorates, the purifying rate ofexhaust gas by the three way catalyst is reduced. In this case, sincethe exhaust gas is purified due to the O₂ storage function of the threeway catalyst, if the O₂ storage function is weakened, the three waycatalyst deteriorates. Accordingly, if the fact that the O₂ storagefunction is weakened can be detected, the deterioration of the three waycatalyst can be detected.

Therefore, in a known engine, a second air-fuel ratio sensor isadditionally arranged in the exhaust passage downstream of the three waycatalyst. The air-fuel mixture fed into the engine cylinder is changedover from a rich mixture to a lean mixture after the air-fuel mixture ismaintained at, for example, a rich mixture for a fixed time. After thechangeover of the air-fuel mixture from the rich mixture to the leanmixture, the air-fuel ratio detected by the second air-fuel ratio sensoris changed from the rich side to the lean side of the stoichiometricair-fuel ratio with a time interval. In this case, if this time intervalis shorter than a predetermined time, it is determined that the threeway catalyst is deteriorated. In this engine, the deterioration of thethree way catalyst is detected by noting the fact that, if the O₂storage function is weakened, when the air-fuel mixture fed into theengine cylinder is changed over, for example, from a rich mixture to alean mixture, the length of time during which the second air-fuel ratiothereafter continues to produce a rich signal indicating that theair-fuel ratio is on the rich side becomes short.

Now, the ability of the three way catalyst for reducing NO_(x) andoxidizing CO and HC decreases as the actual amount of oxygen which canbe stored in the three way catalyst is reduced. Accordingly, the actualamount of oxygen which can be stored in the three way catalyst correctlyrepresents the degree of deterioration of the three way catalyst.Therefore, to correctly detect the degree of deterioration of the threeway catalyst, it is necessary to correctly detect the actual amount ofoxygen which can be stored in the three way catalyst. In this case, theactual amount of oxygen which can be stored in the three way catalystcannot be found based on only the length of time during which the secondair-fuel ratio continues to produce the rich signal in theabove-mentioned engine, and therefore, in the above-mentioned engine, aproblem arises in that it is impossible to correctly detect the degreeof deterioration of the three way catalyst.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a device capable ofcorrectly detecting the degree of deterioration of a catalyst.

According to the present invention, there is provided a device fordetecting the degree of deterioration of a catalyst having an oxygenstorage function, the device comprising: an exhaust gas passage in whichthe catalyst is arranged, an exhaust gas produced by burning fuelflowing within the exhaust gas passage; an air-fuel ratio sensorarranged in the exhaust gas passage downstream of the catalyst to detectan air-fuel ratio; air-fuel ratio changeover means for changing over theair-fuel ratio at the upstream side of the catalyst between apredetermined rich air-fuel ratio and a predetermined lean air-fuelratio; gas amount detecting means for detecting an amount of the exhaustgas passing through the catalyst during a time from when the air-fuelratio is changed over from one of the predetermined rich air-fuel ratioand the predetermined lean air-fuel ratio to the other predeterminedair-fuel ratio to when the air-fuel ratio detected by the air-fuel ratiosensor becomes approximately equal to the other predetermined air-fuelratio; calculating means for calculating an amount of oxygen stored inthe catalyst from the amount of the exhaust gas and a difference betweenthe other predetermined air-fuel ratio and the stoichiometric air-fuelratio; and deterioration determining means for determining a degree ofdeterioration of the catalyst on the basis of the amount of oxygen.

The present invention may be more fully understood from the descriptionof preferred embodiments of the invention set forth below, together withthe accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 is a general view of an engine;

FIGS. 2A and 2B are diagrams illustrating the output of the air-fuelratio sensor;

FIG. 3 is a flow chart showing control of the feedback correctioncoefficient;

FIG. 4 is a diagram illustrating a change in the feedback correctioncoefficient;

FIG. 5 is a diagram illustrating changes in air-fuel ratios detected bythe first air-fuel ratio sensor and the second air-fuel ratio sensor;

FIG. 6 is a diagram illustrating the degree of deterioration of thecatalyst;

FIGS. 7A through 7F are a flow chart for detecting the degree ofdeterioration of the catalyst;

FIGS. 8A through 8G are a flow chart of another embodiment for detectingthe degree of deterioration of the catalyst; and

FIG. 9 is a time chart.

DESCRIPTION OF PREFERRED EMBODIMENTS

Referring to FIG. 1, reference numeral 1 designates an engine body, 2 anintake port and 3 an exhaust port. The intake port 2 is connected to asurge tank 5 via a corresponding branch pipe 4, and the surge tank 5 isconnected to an air cleaner 8 via an intake duct 6 and an air flow meter7. A throttle valve 9 is arranged in the intake duct 6. The exhaust port3 is connected to a catalytic converter 11 containing a three waycatalyst therein via an exhaust manifold 10. A fuel injector 12 which iscontrolled based on a signal output from an electronic control unit 20is arranged in each branch pipe 4.

The electronic control unit 20 comprises a ROM (read only memory) 22, aRAM (random access memory) 23, a CPU (microprocessor etc.) 24, an inputport 25 and an output port 26. The ROM 22, the RAM 23, the CPU 24, theinput port 25 and the output port 26 are interconnected to each othervia a bidirectional bus 21. The air flow meter 7 produces an outputvoltage which is proportional to the amount of air fed into the enginecylinder, and this output voltage is input into the input port 25 via anAD converter 27. An idle switch 13 which is made ON when the throttlevalve 6 is in the idling position is attached to the throttle valve 6,and the output signal of this idle switch 13 is input into the inputport 25. A coolant temperature sensor 14 producing an output voltagewhich is proportional to the temperature of the cooling water of theengine is mounted on the engine body 1, and the output voltage of thecoolant temperature sensor 14 is put into the input port 25 via an ADconverter 28. In addition, an engine speed sensor 15 which produces anoutput pulse representing the engine speed is connected to the inputport 25.

A first air-fuel ratio sensor 16 is arranged in the exhaust passageupstream of the catalytic converter 11, for example, in the exhaustmanifold 10, and a second air-fuel ratio sensor 18 is arranged in theexhaust passage 17 downstream of the catalytic converter 11. The firstair-fuel ratio sensor 16 and the second air-fuel ratio sensor 18 areconnected to the input port 25 via corresponding current-voltageconverting circuits 29, 30 and corresponding AD converters 31, 32. Theoutput port 26 is connected, on one hand, to the fuel injector 12 via adrive circuit 33 and, on the other hand, to a display device 35 fordisplaying the degree of deterioration of the catalyst via a drivecircuit 34. In addition, the electronic control unit 20 furthercomprises a counter 36. This counter 36 is reset by the count resetsignal output to the output port 26 and, once the counter 36 is reset,the counting up operation of the counter 36 is instantaneously started.The count value of this counter 36 is input into the input port 25.

The first air-fuel ratio sensor 16 and the second air-fuel ratio sensor18 have a construction such that the anode is formed on the inner faceof the tubular member made of, for example, zirconia, and the cathode isformed on the outer face of the tubular member, and that the outer faceof the cathode is covered by a porous layer, and a current I whichvaries in accordance with a change in the air-fuel ratio as illustratedin FIG. 2A flows between the anode and the cathode of the first air-fuelratio sensor 16 and between the anode and the cathode of the secondair-fuel ratio sensor 18. This current I is converted to a correspondingvoltage in the corresponding current-voltage converting circuits 29, 30,and an output voltage V which varies in accordance with a change in theair-fuel ratio as illustrated in FIG. 2B is produced at the outputterminals of the current-voltage converting circuits 29, 30.Accordingly, the air-fuel ratio can be detected by the output voltages Vof the current-voltage converting circuits 29, 30.

In the embodiment according to the present invention, the fuel injectiontime TAU of the fuel injector 12 is calculated based on the followingformula.

    TAU=TP·FAF·GA·C·M

where

TP: basic fuel injection time

FAF: feedback correction coefficient

GA: learning coefficient

C: enrichment coefficient

M: air-fuel ratio determining coefficient

The basic fuel injection time TP is a fuel injection time required tomake the air-fuel ratio of air-fuel mixture fed into the engine equal tothe stoichiometric air-fuel ratio, and this basic fuel injection time TPis stored in advance in the ROM 22 as a function of the engine load Q/N(the amount of air Q fed into the engine cylinder/the engine speed N)and the engine speed N.

The feedback correction coefficient FAF is controlled by the outputsignal of the first air-fuel ratio sensor 16 in order to maintain theair-fuel ratio at a target air-fuel ratio. This feedback correctioncoefficient FAF alternately increases and decreases relative to 1.0.

The learning coefficient GA is a coefficient for causing the feedbackcorrection coefficient FAF to increase or decrease relative to 1.0.

The enrichment coefficient C is a coefficient for increasing the amountof fuel to be fed at the time of warm-up of the engine or at the time ofacceleration of the engine. This enrichment coefficient is made 1.0 whenthe increase operation of the amount of fuel is not carried out.

The air-fuel ratio determining coefficient M is a coefficient forobtaining a target air-fuel ratio which is determined in advance inaccordance with the operating state of the engine. The air-fuel ratiodetermining coefficient M is made 1.0 when the target air-fuel ratio isthe stoichiometric air-fuel ratio.

Next, the feedback correction coefficient FAF and the learningcoefficient GA will be briefly described with reference to FIGS. 3 and4. Note that, if the target air-fuel ratio is (A/F)₀, the air-fuel ratiodetermining coefficient M is made the stoichiometric air-fuel ratio/thetarget air-fuel ratio (A/F)₀.

FIG. 3 illustrates a routine processed by sequential interruptions whichare executed at predetermined time intervals.

Referring to FIG. 3, in step 50, the target output voltage E of thecurrent-voltage converting circuit 29 of the first air-fuel ratio sensor16, which voltage E corresponds to the target air-fuel ratio (A/F)₀, iscalculated based on the relationship illustrated in FIG. 2B. Where theair-fuel ratio determining coefficient M is made the stoichiometricair-fuel ratio/the target air-fuel ratio (A/F)₀, if the injection offuel is carried out for the time TP·M, the air-fuel ratio becomesapproximately equal to the target air-fuel ratio (A/F)₀. Accordingly, atthis time, the output voltage of the current-voltage converting circuit29 of the first air-fuel ratio sensor 16 becomes approximately equal tothe target output voltage E.

If the target output voltage E is calculated in step 50, the routinegoes to step 51, and it is determined whether or not the output voltageV of the current-voltage converting circuit 29 of the first air-fuelratio 16 is higher than the target output voltage E, i.e., the air-fuelratio detected by the first air-fuel ratio sensor 16 is on the lean sideof the target air-fuel ratio (A/F)₀. If V>E, i.e., when the air-fuelratio is on the lean side of the target air-fuel ratio (A/F)₀, theroutine goes to step 52, and it is determined whether or not theair-fuel ratio was on the lean side of the target air-fuel ratio (A/F)₀in the previous processing cycle. When the air-fuel ratio was not on thelean side of the target air-fuel ratio (A/F)₀, it is determined that theair-fuel ratio has changed from the rich side to the lean side, and theroutine goes to step 53.

In step 53, the feedback correction coefficient FAF is memorized asFAFR. Then, in step 54, the skip value S is added to FAF, and then theroutine goes to step 55. Conversely, if it is determined in step 52 thatthe air-fuel ratio was also on the lean side of the target air-fuelratio (A/F)₀ in the previous processing cycle, the routine goes to step56, and the integral value K is added to FAF (K<S). Then, the routinegoes to step 55. Accordingly, as illustrated in FIG. 4, when theair-fuel ratio changes from the rich side to the lean side, the feedbackcorrection coefficient FAF is abruptly increased by the skip value S andthen gradually increased.

If it is determined in step 51 that V≦E, i.e., the air-fuel ratio is onthe rich side of the target air-fuel ratio (A/F)₀, the routine goes tostep 57, and it is determined whether or not the air-fuel ratio was onthe rich side of the target air-fuel ratio (A/F)₀ in the previousprocessing cycle. When the air-fuel ratio was not on the rich side, itis determined that the air-fuel ratio has changed from the lean side tothe rich side, and the routine goes to step 58. In step 58, the feedbackcorrection coefficient FAF is memorized as FAFL. Then, in step 59, theskip value S is subtracted from FAF, and then the routine goes to step55. Conversely, if it is determined in step 57 that the air-fuel ratiowas also on the rich side of the target air-fuel ratio (A/F)₀ in theprevious processing cycle, the routine goes to step 60, and the integralvalue K (K<S) is subtracted from FAF. Then, the routine goes to step 55.Accordingly, as illustrated in FIG. 4, when the air-fuel ratio changesfrom the lean side to the rich side, the feedback correction coefficientFAF is abruptly reduced by the skip value S and then gradually reduced.

Then, in step 55, the mean value of FAFL and FAFR is memorized as thelearning coefficient GA. If the feedback correction coefficient FAFbecomes larger than 1.0, since the learning coefficient GA also becomeslarger than 1.0, the value of FAF becomes small. Conversely, if FAFbecomes smaller than 1.0, since GA also becomes smaller than 1.0, thevalue of FAF becomes large. Thus, FAF is caused to alternately increaseand decrease relative to 1.0.

The changing pattern of the feedback correction coefficient FAFillustrated in FIG. 4 does not change even if the target air-fuel ratio(A/F)₀ changes. For example, even if the target air-fuel ratio (A/F)₀ isthe stoichiometric air-fuel ratio, FAF is caused to alternately increaseand decrease relative to 1.0. Accordingly, where the target air-fuelratio (A/F)₀ is the stoichiometric air-fuel ratio, i.e., the air-fuelratio determining coefficient is equal to 1.0, if the value of FAF ismade a fixed value 1.0, i.e., the feedback control of the air-fuel ratiois stopped, the air-fuel ratio is maintained at the stoichiometricair-fuel ratio. Similarly, where the target air-fuel ratio (A/F)₀ is notthe stoichiometric air-fuel ratio, if the air-fuel ratio determiningcoefficient M is made a value corresponding to the target air-fuel ratio(A/F)₀ and, in addition, FAF is made a fixed value 1.0, the air-fuelratio is maintained at the target air-fuel ratio (A/F)₀. Accordingly, tomake the air-fuel ratio equal to the target air-fuel ratio (A/F)₀, it issufficient to merely make the air-fuel ratio determining coefficient M avalue corresponding to the target air-fuel ratio (A/F)₀ and make FAF afixed value 1.0.

A method of detecting the actual amount of oxygen stored in the threeway catalyst will be hereinafter described with reference to FIG. 5. InFIG. 5, the solid line indicates an air-fuel ratio detected by the firstair-fuel ratio sensor 16, and the broken line indicates an air-fuelratio detected by the second air-fuel ratio sensor 18. In addition, FIG.5 illustrates the case where the air-fuel ratio of the air-fuel mixturefed into the engine cylinder is caused to forcibly change over from thelean air-fuel ratio (A/F)_(L) (the air-fuel ratio on the lean side ofthe stoichiometric air-fuel ratio) to the rich air-fuel ratio (A/F)_(R)(the air-fuel ratio on the rich side of the stoichiometric air-fuelratio) at the time t₁, and the air-fuel ratio of the air-fuel mixturefed into the engine cylinder is caused to forcibly change over from therich air-fuel ratio (A/F)_(R) to the lean air-fuel ratio (A/F)_(L) atthe time t₂.

As can be seen from FIG. 5, when the air-fuel ratio of the mixture fedinto the engine cylinder is caused to change over from the lean air-fuelratio (A/F)_(L) to the rich air-fuel ratio (A/F)_(R) at the time t₁, theair-fuel ratio detected by the first air-fuel ratio sensor 16 alsochanges from the lean air-fuel ratio (A/F)_(L) to the rich air-fuelratio (A/F)_(R) and, when the air-fuel ratio of the mixture fed into theengine cylinder is caused to change over from the rich air-fuel ratio(A/F)_(R) to the lean air-fuel ratio (A/F)_(L) at the time t₂, theair-fuel ratio detected by the first air-fuel ratio sensor 16 alsochanges from the rich air-fuel ratio (A/F)_(R) to the lean air-fuelratio (A/F)_(L).

Conversely, as illustrated by the broken line in FIG. 5, the air-fuelratio detected by the second air-fuel ratio sensor 18 changes in apattern which is different from that of the air-fuel ratio detected bythe first air-fuel ratio sensor 16. Namely, when the air-fuel ratio ofthe mixture fed into the engine cylinder is caused to change over fromthe lean air-fuel ratio (A/F)_(L) to the rich air-fuel ratio (A/F)_(R)at the time t₁, the air-fuel ratio detected by the second air-fuel ratiosensor 18 changes from the lean air-fuel ratio (A/F)_(L) to thestoichiometric air-fuel ratio. After this, the air-fuel ratio detectedby the second air-fuel ratio sensor 18 is maintained at thestoichiometric air-fuel ratio for the time ΔT_(R) and then changes tothe rich air-fuel ratio (A/F)_(R). Conversely, when the air-fuel ratioof the mixture fed into the engine cylinder is caused to change overfrom the rich air-fuel ratio (A/F)_(R) to the lean air-fuel ratio(A/F)_(L) at the time t₂, the air-fuel ratio detected by the secondair-fuel ratio sensor 18 changes from the rich air-fuel ratio (A/F)_(R)to the stoichiometric air-fuel ratio. After this, the air-fuel ratiodetected by the second air-fuel ratio sensor 18 is maintained at thestoichiometric air-fuel ratio and then changes to the lean air-fuelratio (A/F)_(L).

It is due to the oxygen storage function of the three way catalyst that,when the air-fuel ratio of the mixture fed into the engine cylinder ischanged over as mentioned above, the air-fuel ratio detected by thesecond air-fuel ratio detector 18 is maintained at the stoichiometricair-fuel ratio for the times ΔT_(R) and ΔT_(L). Namely, when theair-fuel ratio of the mixture fed into the engine cylinder is the leanair-fuel ratio, excess oxygen exists in the exhaust gas, and this excessoxygen is absorbed and stored in the three way catalyst. When theair-fuel ratio of the mixture fed into the engine cylinder is caused tochange over from the lean air-fuel ratio (A/F)_(L) to the rich air-fuelratio (A/F)_(R) at the time t₁, the exhaust gas contains therein theamount of unburned components such as CO, HC, H₂, which amountcorresponds to the air-fuel ratio and, at this time, the oxygen storedin the three way catalyst is used for oxidizing the unburned components.The air-fuel ratio detected by the second air-fuel ratio sensor 18 ismaintained at the stoichiometric air-fuel ratio for the time for whichthe oxidizing operation of the unburned components by the oxygen storedin the three way catalyst is carried out, i.e., for the time ΔT_(R) inFIG. 5. After this, when the oxygen stored in the three way catalystdisappears, the oxidizing operation of the unburned components is nolonger carried out, and thus the air-fuel ratio detected by the secondair-fuel ratio sensor 18 changes to the rich air-fuel ratio (A/F)_(R).

After this, when the air-fuel ratio of the mixture fed into the enginecylinder is caused to change over from the rich air-fuel ratio (A/F)_(R)to the lean air-fuel ratio (A/F)_(L) at the time t₂, the absorbingoperation of oxygen by the three way catalyst is started. The air-fuelratio detected by the second air-fuel ratio sensor 18 is maintained atthe stoichiometric air-fuel ratio for the time for which the absorbingoperation of oxygen is carried out, i.e., for the time ΔT_(R) in FIG. 5.After this, when the absorbing ability of oxygen of the three waycatalyst is saturated, oxygen is no longer absorbed in the three waycatalyst, and thus the air-fuel ratio detected by the second air-fuelratio sensor 18 changes to the lean air-fuel ratio (A/F)_(L). Sinceoxygen in the exhaust gas is taken away by the three way catalyst forthe time for which the absorbing operation of oxygen is carried out, theunburned components such as HC, CO, H₂ in the exhaust gas take awayoxygen from NO_(x), and, as a result, NO_(x) is reduced. After this,when the absorbing ability of oxygen of the three way catalyst issaturated, the unburned components in the exhaust gas are oxidized byoxygen contained in the exhaust gas. As a result, since the reducingoperation of NO_(x) is no longer carried out, NO_(x) is discharged fromthe three way catalyst.

The upper limit exists in the actual amount of oxygen which the threeway catalyst is able to absorb and store therein and, if the type andsize of the three way catalyst is determined, the actual amount ofoxygen which the three way catalyst is able to absorb and store thereinis accordingly determined. In this case, the amount of unburnedcomponents which can be oxidized and the amount of NO_(x) which can bereduced are decreased as the actual amount of oxygen which the three waycatalyzer is able to absorb and store therein is decreased. Accordingly,the purifying efficiency of the exhaust gas becomes low as the actualamount of oxygen which the three way catalyst is able to absorb andstore therein is decreased. Whereas, if the three way catalystdeteriorates, the amount of the unburned components which can beoxidized and the amount of NO_(x) which can be reduced are decreased,and thus the purifying efficiency of the exhaust gas becomes low.Accordingly, the actual amount of oxygen which the three way catalyst isable to absorb and store therein properly represents the degree ofdeterioration of the three way catalyst. FIG. 7 illustrates therelationship between the degree of deterioration of the three waycatalyst and the actual amount of oxygen OS which can be absorbed andstored in the three way catalyst.

Accordingly, if the actual amount of oxygen which can be absorbed andstored in the three way catalyzer is detected, it is possible tocorrectly detect the degree of deterioration of the three way catalyst.

Assuming that the amount of air Go(g) is fed into the engine and, atthis time, the air-fuel ratio of the mixture fed into the enginecylinder becomes equal to the rich air-fuel ratio (A/F)_(R) illustratedin FIG. 5. At this time, the amount of air which is short is representedby [Stoichiometric air-fuel ratio-(A/F)_(R) ]·Go(g). In this case, if[Stoichiometric air-fuel ratio-(A/F)_(R) ] is represented by Δ(A/F)_(R),the amount of air which is short is represented by Δ(A/F)_(R) ·Go(g). Inaddition, at this time, if the ratio of the amount of oxygen containedin air is represented by α, the amount of oxygen which is short isrepresent by α·Δ(A/F)_(R) ·Go(g). This amount of oxygen α·Δ(A/F)_(R)·Go(g) which is short is released from the three way catalyst during thetime AT_(R) in FIG. 5, and thus the amount of oxygen which is stored inthe three way catalyst becomes equal to α·Δ(A/F)_(R) ·Go(g).Accordingly, if the amount of air Go which is fed into the enginecylinder during the time AT_(R) is detected, the amount of oxygen whichis stored in the three way catalyst can be found.

Where the air-flow meter 7 is used for detecting the amount of air fedinto the engine cylinder as illustrated in FIG. 1, the air flow meter 7produces an output voltage which is proportional to the amount of airGa(g/sec) fed into the engine cylinder per a unit time. Accordingly, ifthe amount of air Ga(g/sec) detected by the air flow meter 7 ismultiplied by ΔT_(R), the resultant Ga·ΔT_(R) indicates the amount ofair Go which is fed into the engine cylinder during the time ΔT_(R).Accordingly, where the air flow meter 7 is used, the amount of oxygenwhich can be absorbed and stored in the three way catalyst isrepresented by α·Δ(A/F)_(R) ·Ga·ΔAT_(R). In this case, the amount of airGa. ΔTa fed into the engine cylinder is approximately equal to theamount of exhaust gas passing through the three way catalyst, andΔ(A/F)_(R) indicates a difference in the air-fuel ratio relative to thestoichiometric air-fuel ratio. Accordingly, in other words, the amountof oxygen which can be absorbed and stored in the three way catalyst canbe obtained from the amount of exhaust gas passing through the three waycatalyst and the difference in the air-fuel ratio relative to thestoichiometric air-fuel ratio.

Conversely, assuming that the amount of air Go(g) is fed into the enginecylinder and, at this time, the air-fuel ratio of the mixture fed intothe engine cylinder becomes equal to the lean air-fuel ratio (A/F)_(L)illustrated in FIG. 5, at this time, the amount of excess air isrepresented by [(A/F)_(L) -stoichiometric air-fuel ratio]·Go(g). In thiscase, if [(A/F)_(L) -stoichiometric air-fuel ratio] is represented byΔ(A/F)_(L), the amount of excess air is represented by Δ(A/F)_(L)·Go(g). Accordingly, if the above-mentioned α is used, the amount ofexcess oxygen is represented by α·Δ(A/F)_(L) ·Go(g). The amount ofexcess oxygen α·Δ(A/F)_(L) ·Go(g) is stored in the three way catalystduring the time ΔT_(L) in FIG. 5. Accordingly, the actual amount ofoxygen which can be stored in the three way catalyst becomes equal toα·Δ(A/F)_(L) ·Go(g).

Accordingly, if the above-mentioned Ga and ΔT_(L) illustrated in FIG. 5are used, the actual amount of oxygen which can be stored in the threeway catalyst is represented by α·Δ(A/F)_(L) ·Ga·ΔT_(L). Since the actualamount of air obtained based on Δ(A/F)_(L) and ΔT_(L) becomes equal tothe actual amount of air obtained based on Δ(A/F)_(R) and ΔT_(R), ifΔ(A/F)_(L) is made equal to Δ(A/F)_(R) in FIG. 5, ΔT_(L) becomes equalto ΔT_(L).

As mentioned above, the actual amount of oxygen which can be absorbedand stored in the three way catalyst is represented by α·Δ(A/F)_(R)·Ga·ΔT_(R) or α·Δ(A/F)_(L) ·Ga·ΔT_(L). In this case, if the air-fuelratio determining coefficient M is made, for example, a valuecorresponding to the rich air-fuel ratio (A/F)_(R), since the air-fuelratio of the mixture fed into the engine cylinder becomes equal to therich air-fuel ratio (A/F)_(R) as mentioned above, it is possible toobtain Δ(A/F)_(R) and Δ(A/F)_(L) from the air-fuel ratio determiningcoefficient M. In this case, naturally, Δ(A/F)_(R) and Δ(A/F)_(L) can beobtained by the output signal of the first air-fuel ratio sensor 16. Inaddition, α is known, and Ga can be obtained from the output signal ofthe air flow meter 7. Furthermore, ΔT_(R) can be obtained by detectingthe length of time between the time t₁ and the time at which theair-fuel ratio detected by the second air-fuel ratio sensor 18 becomesthe rich air-fuel ratio (A/F)_(R), and ΔT_(L) can be obtained bydetecting the length of time between the time t₂ and the time at whichthe air-fuel ratio detected by the second air-fuel ratio sensor 18becomes the lean air-fuel ratio (A/F)_(L).

In this case, it can be determined whether the air-fuel ratio detectedby the second air-fuel ratio sensor 18 becomes the rich air-fuel ratio(A/F)_(R) or the lean air-fuel ratio (A/F)_(L) by determining whether ornot the air-fuel ratio detected by the second air-fuel ratio sensor 18becomes equal to the air-fuel ratio detected by the first air-fuel ratiosensor 16, or whether or not the air-fuel ratio detected by the secondair-fuel ratio sensor 18 becomes equal to the target air-fuel ratiodetermined by the air-fuel ratio determining coefficient M. Accordingly,α·Δ(A/F)_(R) ·Ga·ΔT_(R) and α·Δ(A/F)_(L) ·Ga·ΔT_(L) can be calculated.This is the first method for detecting the actual amount of oxygen whichcan be absorbed and stored in the three way catalyst.

Where ΔT_(R) or ΔT_(L) is detected by determining whether or not theair-fuel ratio detected by the second air-fuel ratio sensor 18 becomesequal to the target air-fuel ratio determined by the air-fuel ratiodetermining coefficient M as mentioned above, it is not necessary toarrange the first air-fuel ratio sensor 16 from the point of view ofdetecting the actual amount of oxygen, and it is enough to arrange onlythe second air-fuel ratio 18.

The second method for detecting the actual amount of oxygen which can beabsorbed and stored in the three way catalyst is a method of obtainingGa, Δ(A/F)_(R) and Δ(A/F)_(L) many times during the ΔT_(R) and ΔT_(L).Namely, in this second method, the times ΔT_(R) and ΔT_(L) are dividedinto continuous times Δt₁, Δt₂, . . . Δt_(n), and the actual amount ofoxygen ΔOS(=α·Δ(A/F)_(R) ·Ga·Δt or α·Δ(A/F)_(L) ·Ga·Δt) which is storedin the three way catalyst during each time Δt is obtained. Then, thetotal actual amount of oxygen which can be stored in the three waycatalyst is obtaining by summing up all the ΔOS. This second method hasan advantage that it is possible to correctly detect the entire actualamount of oxygen which can be stored in the three way catalyst even ifthe amount of air Ga changes during the time ΔT_(R) or ΔT_(L) , or evenif Δ(A/F)_(R) or Δ(A/F)_(L) changes although they actually changelittle.

FIGS. 7A through 7F illustrate a routine which is repeatedly executedfor carrying out the above-mentioned first method, and FIGS. 8A through8G illustrate a routine which is repeatedly executed for carrying outthe above-mentioned second method. In addition, FIG. 9 illustrates atime chart common to both routines.

Referring to FIGS. 7A through 7F, in step 100, it is determined whetheror not the condition for executing the detection of deterioration of thecatalyst stands. For example, when the temperature of the cooling waterof the engine is higher than a predetermined temperature, when both theair-fuel ratio sensors 16, 18 produce a regular output signal, and whenthe operating state of the engine is not an idling state, it isdetermined that the executing condition stands. When the executingcondition does not stand, flags X1, X2, X3, E1, E2, E3, END1, END2, Z1,Z2 and W which are used in this routine are reset in steps 101, 102, 103and 104. Then, the processing cycle is completed.

If the executing condition stands, the routine goes from step 100 tostep 110, and it is determined whether or not the flag X1 is set. Atthis time, since the flag X1 is reset, the routine goes to step 130, andit is determined whether or not the flag X2 is set. At this time, sincethe flag X2 is reset, the routine goes to step 150, and it is determinedwhether or not the flag X3 is set. At this time, since the flag X3 isreset, the routine goes to step 170. In step 170, the flag X1 indicatingthat the first stage should be executed is set. Then, the routine goesto step 171 shown in FIG. 7F, and it is determined whether or not theflag END1 is set. At this time, since the flag END1 is reset, theroutine goes to step 181, and it is determined whether or not the flagEND2 is set. At this time, since the flag END2 is reset, the processingcycle is completed.

In the next processing cycle, since it is determined in step 110, shownin FIG. 7A, that the flag X1 is set, the routine goes to step 111 shownin FIG. 7B. In step 111, it is determined whether or not the flag E1indicating that the first stage is being executed is set. At this time,since the flag E1 is reset, the routine goes to step 112. In step 112,the air-fuel ratio A/F of the mixture fed into the engine cylinder ismade the predetermined lean air-fuel ratio (A/F)_(L). Namely, theair-fuel ratio determining coefficient M is made a value correspondingto the lean air-fuel ratio (A/F)_(L), and the feedback correctioncoefficient FAF is made a fixed value 1.0. Then, in step 113, the flagE1 is set, and then the routine goes to step 114. In step 114, a dataindicating that the counter 36 is to be reset is output to the outputport 26, and thereby the count value of the counter 36 is made zero.When the counter 36 is reset, the counting up operation of the counter36 is instantaneously started.

Then, in step 115, the air-fuel ratio (A/F)_(in) detected by the firstair-fuel ratio sensor 16 is read in, and then, in step 116, the air-fuelratio (A/F)_(out) detected by the second air-fuel ratio sensor 18 isread in. Then, in step 117, it is determined whether or not the countvalue C exceeds a predetermined value A. This value A is stored inadvance in the ROM 22 as a function of the lean air-fuel ratio (A/F)_(L)and the amount of air Ga detected by the air-flow meter 7. When theroutine goes to step 117 for the first time, since the count value C islower than the value A, the routine jumps to step 171 shown in FIG. 7F,and then the processing cycle is completed via step 181. Note that thepredetermined value A is determined so that oxygen can be absorbed andstored in the three way catalyst to the absorption limit by the time thecount C reaches the value A.

In the next processing cycle, the routine jumps from step 111 to step115. When the count value C becomes equal to the value A, the routinegoes from step 117 to step 118, and the flag X1 is reset. Then, in steps119, the flag E1 is reset. Then, in step 120, the flag X2 indicatingthat the second stage should be executed is set, and then the processingcycle is completed.

If the flag X1 is reset, and the flag X2 is set, the routine goes fromstep 130 shown in FIG. 7A to step 131 shown in FIG. 7C in the nextprocessing cycle. In the step 131, the amount of air Ga detected by theair flow meter 7 is read in. Then, in step 132, it is determined whetheror not the flag E2 indicating that the second stage is being executed isset. At this time, since the flag E2 is reset, the routine goes to step133. In step 133, the air-fuel ratio A/F of the mixture fed into theengine cylinder is made a predetermined rich air-fuel ratio (A/F)_(R).Namely, the air-fuel ratio determining coefficient M is made a valuecorresponding to the rich air-fuel ratio (A/F)_(R), and the feedbackcorrection coefficient FAF is made a fixed value 1.0. Accordingly, ascan be seen from FIG. 9, when the count value C reaches thepredetermined value A, the air-fuel ratio of the mixture fed into theengine is changed over from the lean air-fuel ratio (A/F)_(L) to therich air-fuel ratio (A/F)_(R). Then, in step 134, the flag E2 is set,and then the routine goes to step 135. In step 135, a data indicatingthat the counter 36 is to be reset is output to the output port 26, andthereby the count value C of the counter 36 is made zero. When thecounter 36 is reset, the counting up operation of the counter 36 isinstantaneously started.

Then, in step 136, the air-fuel ratio (A/F)_(in) detected by the firstair-fuel ratio sensor 16 is read in, and then in step 137, the air-fuelratio (A/F)_(out) detected by the second air-fuel ratio sensor 18 isread in. Then, in step 138, it is determined whether or not the air-fuelratio (A/F)_(in) detected by the first air-fuel ratio sensor 16 becomesapproximately equal to the air-fuel ratio (A/F)_(out) detected by thesecond air-fuel ratio sensor 18. At this time, since (A/F)_(in) is notequal to (A/F)_(out), the routine jumps to step 171 shown in FIG. 7F.Then, the processing cycle is completed via step 181.

As illustrated in FIG. 9, (A/F)_(in) becomes approximately equal to(A/F)_(out) a little while after the air-fuel ratio of the mixture ischanged over from the lean air-fuel ratio (A/F)_(L) to the rich air-fuelratio. If (A/F)_(in) becomes approximately equal to (A/F)_(out), theroutine goes from step 138 to step 139, and it is determined whether ornot the flag W is set. At this time, since the flag W is reset, theroutine goes to step 140, and the count value C is made C1. Accordingly,this C1 represents ΔT_(R) in FIG. 5. Then, in step 141, the counter 36is reset, and then, in step 142, the flag END1 is set. Then, in step143, the flag W determining a waiting time until the third stage isstarted is set, and then the routine goes to step 171 shown in FIG. 7F.

In step 171, since it is determined that the flag END1 is set, theroutine goes to step 172, and the actual amount of oxygen OS1 which isstored in the three way catalyst is found. As mentioned above, this OS1is calculated from α·Δ(A/F)_(R) ·Ga·ΔT_(R). In this case, α is a fixedvalue, and Δ(A/F)_(R) is calculated from a difference between thestoichiometric air-fuel ratio and (A/F)_(in) detected by the firstair-fuel ratio sensor 16. In addition, Ga is calculated from the outputsignal of the air flow meter 7, and ΔT_(R) is calculated from the countvalue C1.

If the actual amount of oxygen OS1 is calculated, the routine goes tostep 173, and the flag END1 is reset. Then, in step 174, the flag Z1indicating that the calculation of OS1 is completed is set. Then, instep 175, it is determined whether or not the flag Z1 is set. At thistime, since the flag Z is set, the routine goes to step 176, and it isdetermined whether or not the flag Z2 is set. At this time, since theflag Z2 is reset, the processing cycle is completed.

In the next processing cycle, since it is determined in step 139 in FIG.7D that the flag W is set, the routine goes to step 144. In step 144, itis determined whether or not the count value C exceeds a predeterminedvalue B. This value B is stored in advance in the ROM 22 as a functionof the lean air-fuel ratio (A/F)_(L) and the amount of air Ga detectedby the air flow meter 7. When the routine goes to step 144 for the firsttime, since the count value C is lower than the value B, the routinejumps to step 171 shown in FIG. 7F. Then, the processing cycle iscompleted via step 181. Note that the predetermined value B isdetermined so that the oxygen stored in the three way catalyst can becompletely released by the time the count value C reaches the value B,in order to improve the accuracy in the following judgement.

When the count value C becomes equal to the predetermined value B, theroutine goes from step 144 to step 145, the flag W is reset. Then, instep 146, the flag X2 is reset, and then in step 147, the flag E2 isreset. Then, in step 148, the flag X3 indicating that the third stageshould be executed is set, and then the processing cycle is completed.

If the flag X2 is reset, and the flag X3 is set, the routine goes tostep 150 shown in FIG. 7A to step 151 shown in FIG. 7E. In step 151, itis determined whether or not the flag E3 indicating that the third stageis being executed is set. At this time, since the flag E3 is reset, theroutine goes to step 152. In step 152, the air-fuel ratio A/F of themixture fed into the engine cylinder is made a predetermined leanair-fuel ratio (A/F)_(L). Namely, the air-fuel ratio determiningcoefficient M is made a value corresponding to the lean air-fuel ratio(A/F)_(L), and the feedback correction coefficient FAF is made a fixedvalue 1.0. Then, in step 153, the flag E3 is set, and then the routinegoes to step 154. In step 154, a data indicating that the counter 36 isto be reset is output into the output port 26, and the count value C ofthe counter 36 is made zero.

Then, in step 155, the air-fuel ratio (A/F)_(in) detected by the firstair-fuel ratio sensor 16 is read in, and then in step 156, the air-fuelratio (A/F)_(out) detected by the second air-fuel ratio sensor 18 isread in. Then, in step 157, it is determined whether or not the air-fuelratio (A/F)_(in) detected by the first air-fuel ratio sensor 16 becomesapproximately equal to the air-fuel ratio (A/F)_(out) detected by thesecond air-fuel sensor 18. At this time, since (A/F)_(in) is not equalto (A/F)_(out), the routine jumps to step 171. Then, the processingcycle is completed via step 181.

As illustrated in FIG. 9, (A/F)_(in) becomes approximately equal to(A/F)_(out) a little while after the air-fuel ratio of the mixture ischanged over from the rich air-fuel ratio (A/F)_(R) to the lean air-fuelratio (A/F)_(L). If (A/F)_(in) becomes approximately equal to(A/F)_(out), the routine goes from step 157 to step 158, and the countvalue C is made C2. Accordingly, this C2 represents ΔT_(L) in FIG. 5.Then, in step 159, the counter 36 is reset, and then in step 160, theflag END2 is set. Then, in step 161, the flag X3 is reset, and then instep 162, the flag E3 is reset. Then, the routine goes to step 181 viastep 171.

In step 181, since it is determined that the flag END2 is set, theroutine goes to step 182, and the actual amount of oxygen OS2 which isstored in the three way catalyst is found. As mentioned above, this OS2is calculated from α·Δ(A/F)_(L) ·Ga·ΔT_(L). In this case, α is a fixedvalue, and Δ(A/F)_(L) is calculated from a difference between thestoichiometric air-fuel ratio and (A/F)_(in) detected by the firstair-fuel ratio sensor 16. In addition, Ga is calculated from the outputsignal of the air flow meter 7, and ΔT_(L) is calculated from the countvalue C2.

If the actual amount of oxygen OS2 is calculated, the routine goes tostep 183, and the flag END2 is reset. Then, in step 184, the flag Z2indicating that the calculation of OS2 is completed is set. Then, instep 175, it is determined whether or not the flag Z1 is set. At thistime, since the flag Z1 is set, the routine goes to step 176, and it isdetermined whether or not the flag Z2 is set. At this time, since theflag Z2 is set, the routine goes to step 177.

In step 177, the mean value of OS1 and OS2 is made OS. Then, in step178, the degree of deterioration of the three way catalyst is calculatedfrom the relationship illustrated in FIG. 6. Then, in step 179, thedegree of deterioration of the three way catalyst is displayed by thedisplay device 35. In this case, the degree of deterioration of thethree way catalyst thus calculated may be stored in a so called back-upRAM (not shown) to which a power continues to be supplied from a batteryeven if the engine is stopped, to retain the information to bememorized. Then, in step 180, the flags Z1 and Z2 are reset.

As mentioned above, FIGS. 8A through 8G illustrate the routine forcarrying out the second method.

Referring to FIGS. 8A through 8G, in step 200, it is determined whetheror not the condition for executing the detection of deterioration of thecatalyst stands. As mentioned above, for example, when the temperatureof the cooling water of the engine is higher than a predeterminedtemperature, when both the air-fuel ratio sensors 16, 18 produce aregular output signal, and when the operating state of the engine is notan idling state, it is determined that the executing condition stands.When the executing condition does not stand, flags X1, X2, X3, E1, E2,E3, END1, END2, Z1, Z2 and W which are used in this routine are reset insteps 201, 202, 203 and 204, and then in step 205, OS1, OS2, ΔOS arecleared. Then, the processing cycle is completed.

If the executing condition stands, the routine goes from step 200 tostep 210, and it is determined whether or not the flag X1, is set. Atthis time, since the flag X1 is reset, the routine goes to step 230, andit is determined whether or not the flag X2 is set. At this time, sincethe flag X2 is reset, the routine goes to step 260, and it is determinedwhether or not the flag X3 is set. At this time, since the flag X3 isreset, the routine goes to step 280. In step 280, the flag X1 indicatingthat the first stage should be executed is set. Then, the routine goesto step 281 shown in FIG. 8G, and it is determined whether or not theflag END1 is set. At this time, since the flag END1 is reset, theroutine goes to step 290, and it is determined whether or not the flagEND2 is set. At this time, since the flag END2 is reset, the processingcycle is completed.

In the next processing cycle, since it is determined in step 210, shownin FIG. 8A, that the flag X1 is set, the routine goes to step 211 shownin FIG. 8B. In step 211, it is determined whether or not the flag E1indicating that the first stage is being executed is set. At this time,since the flag E1 is reset, the routine goes to step 212. In step 212,the air-fuel ratio A/F of the mixture fed into the engine cylinder ismade the predetermined lean air-fuel ratio (A/F)_(L). Namely, theair-fuel ratio determining coefficient M is made a value correspondingto the lean air-fuel ratio (A/F)_(L), and the feedback correctioncoefficient FAF is made a fixed value 1.0. Then, in step 213, the flagE1 is set, and then the routine goes to step 214. In step 214, a dataindicating that the counter 36 is to be reset is output to the outputport 26, and thereby the count value of the counter 36 is made zero.When the counter 36 is reset, the counting up operation of the counter36 is instantaneously started.

Then, in step 215, the air-fuel ratio (A/F)_(in) detected by the firstair-fuel ratio sensor 16 is read in, and then, in step 216, the air-fuelratio (A/F)_(out) detected by the second air-fuel ratio sensor 18 isread in. Then, in step 217, it is determined whether or not the countvalue C exceeds a predetermined value A. This value A is stored inadvance in the ROM 22 as a function of the lean air-fuel ratio (A/F)_(L)and the amount of air Ga detected by the air-flow meter 7. When theroutine goes to step 217 for the first time, since the count value C islower than the value A, the routine jumps to step 281 shown in FIG. 8G,and then the processing cycle is completed via step 290.

In the next processing cycle, the routine jumps from step 211 to step215. When the count value C becomes equal to the value A, the routinegoes from step 217 to step 218, and the flag X1 is reset. Then, in step219, the flag E1 is reset. Then, in step 220, the flag X2 indicatingthat the second stage should be executed is set, and then the processingcycle is completed.

If the flag X1 is reset, and the flag X2 is set, the routine goes fromstep 230 shown in FIG. 8A to step 231 shown in FIG. 8C in the nextprocessing cycle. In step 231, it is determined whether or not the flagE2 indicating that the second stage is being executed is set. At thistime, since the flag E2 is reset, the routine goes to step 232. In step232, the air-fuel ratio A/F of the mixture fed into the engine cylinderis made a predetermined rich air-fuel ratio (A/F)_(R). Namely, theair-fuel ratio determining coefficient M is made a value correspondingto the rich air-fuel ratio (A/F)_(R), and the feedback correctioncoefficient FAF is made a fixed value 1.0. Accordingly, as can be seenfrom FIG. 9, when the count value C reaches the predetermined value A,the air-fuel ratio of the mixture fed into the engine is changed overfrom the lean air-fuel ratio (A/F)_(L) to the rich air-fuel ratio(A/F)_(R). Then, in step 233, the flag E2 is set, and then the routinegoes to step 234. In step 234, a data indicating that the counter 36 isto be reset is output to the output port 26, and thereby the count valueC of the counter 36 is made zero. When the counter 36 is reset, thecounting up operation of the counter 36 is instantaneously started.

Then, in step 235, the air-fuel ratio (A/F)_(in) detected by the firstair-fuel ratio sensor 16 is read in, and then in step 236, the air-fuelratio (A/F)_(out) detected by the second air-fuel ratio sensor 18 isread in. Then, in step 237, the amount of air Ga detected by the airflow meter 7 is read in. Then, in step 238, K·(C-C') is made At, andthen in step 139, the count value C is made C'. Namely, in step 238, thecount value C' in the previous processing cycle is subtracted from thecurrent count value C, and At is calculated by multiplying the result ofsubtraction (C-C') by a coefficient K used for converting the countvalue to time. Accordingly, At indicates a length of time from when Atwas calculated in the previous processing cycle to when At is calculatedin the current processing cycle.

Then, in step 240, the actual amount of oxygen which is absorbed andstored in the three way catalyst during the At is calculated. This ΔOSis calculated from α·Δ(A/F)_(R) ·Ga·Δt. In this case, α is a fixedvalue, and Δ(A/F)_(R) is calculated from a difference between thestoichiometric air-fuel ratio and (A/F)_(in) detected by the firstair-fuel ratio sensor 16. In addition, Ga has been calculated from theoutput signal of the air flow meter 7 in step 237, and At has beencalculated based on the count value C in step 238. Then, in step 241,ΔOS is added to OS1. Then, the routine goes to step 242 in FIG. 8D. Instep 242, it is determined whether or not the air-fuel ratio (A/F)_(in)detected by the first air-fuel ratio sensor 16 becomes approximatelyequal to the air-fuel ratio (A/F)_(out) detected by the second air-fuelratio sensor 18. At this time, since (A/F)_(in) is not equal to(A/F)_(out), the routine jumps to step 281 shown in FIG. 8G. Then, theprocessing cycle is completed via step 290.

As illustrated in FIG. 9, (A/F)_(in) becomes approximately equal to(A/F)_(out) a little while after the air-fuel ratio of the mixture ischanged over from the lean air-fuel ratio (A/F)_(L) to the rich air-fuelratio. If (A/F)_(in) becomes approximately equal to (A/F)_(out), theroutine goes from step 242 to step 243, and it is determined whether ornot the flag W is set. At this time, since the flag W is reset, theroutine goes to step 244. In step 244, AS is made zero, and thus thecalculation of OS1 is completed. Accordingly, OS1 represents the actualamount of oxygen which has been stored in the three way catalyst duringΔT_(R) in FIG. 5. Then, in step 245, the counter 36 is reset, and then,in step 246, the flag END1 is set. Then, in step 247, the flag Wdetermining a waiting time until the third stage is started is set, andthen the routine goes to step 281 shown in FIG. 8G.

In step 281, since it is determined that the flag END1 is set, theroutine goes to step 282, and the flag END1 is reset. Then, in step 283,the flag Z1 indicating that the calculation of OS1 is completed is set.Then, in step 284, it is determined whether or not the flag Z1 is set.At this time, since the flag Z is set, the routine goes to step 285, andit is determined whether or not the flag Z2 is set. At this time, sincethe flag Z2 is reset, the processing cycle is completed.

In the next processing cycle, since it is determined in step 243 in FIG.8D that the flag W is set, the routine goes to step 248. In step 248, itis determined whether or not the count value C exceeds a predeterminedvalue B. This value B is stored in advance in the ROM 22 as a functionof the lean air-fuel ratio (A/F)_(L) and the amount of air Ga detectedby the air flow meter 7. When the routine goes to step 248 for the firsttime, since the count value C is lower than the value B, the routinejumps to step 281 shown in FIG. 8G. Then, the processing cycle iscompleted via step 290.

When the count value C becomes equal to the predetermined value B, theroutine goes from step 248 to step 249, the flag W is reset. Then, instep 250, the flag X2 is reset, and then in step 251, the flag E2 isreset. Then, in step 252, the flag X3 indicating that the third stageshould be executed is set, and then the processing cycle is completed.

If the flag X2 is reset, and the flag X3 is set, the routine goes tostep 260 shown in FIG. 8A to step 261 shown in FIG. 8E. In step 261, itis determined whether or not the flag E3 indicating that the third stageis being executed is set. At this time, since the flag E3 is reset, theroutine goes to step 162. In step 162, the air-fuel ratio A/F of themixture fed into the engine cylinder is made a predetermined leanair-fuel ratio (A/F)_(L). Namely, the air-fuel ratio determiningcoefficient M is made a value corresponding to the lean air-fuel ratio(A/F)_(L), and the feedback correction coefficient FAF is made a fixedvalue 1.0. Then, in step 263, the flag E3 is set, and then the routinegoes to step 264. In step 264, a data indicating that the counter 36 isto be reset is output into the output port 26, and the count value C ofthe counter 36 is made zero.

Then, in step 265, the air-fuel ratio (A/F)_(in) detected by the firstair-fuel ratio sensor 16 is read in, and then in step 266, the air-fuelratio (A/F)_(out) detected by the second air-fuel ratio sensor 18 isread in. Then, in step 267, the amount of air Ga detected by the airflow meter 7 is read in. Then, in step 268, K·(C-C') is made at, andthen in step 269, the count value C is made C'. Namely, in step 268, thecount value C' in the previous processing cycle is subtracted from thecurrent count value C, and At is calculated by multiplying the result ofsubtraction (C-C') by a coefficient K used for converting the countvalue to time. Accordingly, At indicates a length of time from when Atwas calculated in the previous processing cycle to when At is calculatedin the current processing cycle.

Then, in step 270, the actual amount of oxygen ΔOS which is absorbed andstored in the three way catalyst during the at is calculated. This ΔOSis calculated from α·Δ(A/F)_(L) ·Ga·Δt. In this case, α is a fixedvalue, and Δ(A/F)_(L) is calculated from a difference between thestoichiometric air-fuel ratio and (A/F)_(in) detected by the firstair-fuel ratio sensor 16. In addition, Ga has been calculated from theoutput signal of the air flow meter 7 in step 267, and At has beencalculated based on the count value C in step 268. Then, in step 271,ΔOS is added to OS2. Then, the routine goes to step 272 in FIG. 8F. Instep 272, it is determined whether or not the air-fuel ratio (A/F)_(in)detected by the first air-fuel ratio sensor 16 becomes approximatelyequal to the air-fuel ratio (A/F)_(out) detected by the second air-fuelsensor 18. At this time, since (A/F)_(in) is not equal to (A/F)_(out),the routine jumps to step 281. Then, the processing cycle is completedvia step 290.

As illustrated in FIG. 9, (A/F)_(in) becomes approximately equal to(A/F)_(out) a little while after the air-fuel ratio of the mixture ischanged over from the rich air-fuel ratio (A/F)_(R) to the lean air-fuelratio (A/F)_(L). If (A/F)_(in) becomes approximately equal to(A/F)_(out), the routine goes from step 272 to step 273. In step 273, ΔSis made zero, and thus, the calculation of OS2 is completed.Accordingly, OS2 represents the actual amount of oxygen which is storedin the three way catalyst during ΔT_(L). Then, in step 274, the counter36 is reset, and then in step 275, the flag END2 is set. Then, in step276, the flag X3 is reset, and then in step 277, the flag E3 is reset.Then, the routine goes to step 290 via step 281.

In step 290, since it is determined that the flag END2 is set, theroutine goes to step 291, and the flag END2 is reset. Then, in step 292,the flag Z2 indicating that the calculation of OS2 is completed is set.Then, in step 284, it is determined whether or not the flag Z1 is set.At this time, since the flag Z1 is set, the routine goes to step 285,and it is determined whether or not the flag Z2 is set. At this time,since the flag Z2 is set, the routine goes to step 286.

In step 286, the mean value of OS1 and OS2 is made OS. Then, in step 287the degree of deterioration of the three way catalyst is calculated fromthe relationship illustrated in FIG. 6. Then, in step 288, the degree ofdeterioration of the three way catalyst is displayed by the displaydevice 35. Then, in step 289, the flags Z1 and Z2 are reset.

According to the present invention, since the actual amount of oxygenwhich can be absorbed and stored in the catalyst can be detected, it ispossible to correctly detect the degree of deterioration of thecatalyst.

While the invention has been described by reference to specificembodiments chosen for purposes of illustration, it should be apparentthat numerous modifications could be made thereto by those skilled inthe art without departing from the basic concept and scope of theinvention.

We claim:
 1. A device for detecting the degree of deterioration of acatalyst having an oxygen storage function, said device comprising:anexhaust gas passage in which the catalyst is arranged, wherein anexhaust gas is produced by burning fuel flowing within said exhaust gaspassage; an air-fuel ratio sensor arranged in said exhaust gas passagedownstream of the catalyst to detect an air-fuel ratio; air-fuel ratiochangeover means for changing over the air-fuel ratio at an upstreamside of the catalyst between a predetermined rich air-fuel ratio and apredetermined lean air-fuel ratio; gas amount detecting means fordetecting an amount of the exhaust gas passing through the catalystduring a time from when the air-fuel ratio is changed over from one ofsaid predetermined rich air-fuel ratio and said predetermined leanair-fuel ratio to the other predetermined air-fuel ratio to when theair-fuel ratio detected by said air-fuel ratio sensor becomesapproximately equal to the other predetermined air-fuel ratio;calculating means for calculating an amount of oxygen stored in thecatalyst based on said detected amount of the exhaust gas and adifference between the other predetermined air-fuel ratio and astoichiometric air-fuel ratio; and deterioration determining means fordetermining a degree of deterioration of the catalyst on the basis ofsaid calculated amount of oxygen stored in the catalyst.
 2. A deviceaccording to claim 1, further comprising an additional air-fuel ratiosensor arranged in said exhaust gas passage upstream of the catalyst todetect an air-fuel ratio, said gas amount detecting means detecting theamount of the exhaust gas passing through the catalyst during a timefrom when the air-fuel ratio is changed over from one of saidpredetermined rich air-fuel ratio and said predetermined lean air-fuelratio to the other predetermined air-fuel ratio to when the air-fuelratio detected by said air-fuel ratio sensor arranged downstream of thecatalyst becomes approximately equal to the air-fuel ratio detected bysaid additional air-fuel ratio sensor, said calculating meanscalculating the amount of oxygen stored in the catalyst from said amountof the exhaust gas and a difference between the stoichiometric air-fuelratio and the air-fuel ratio detected by said additional air-fuel ratiosensor.
 3. A device according to claim 1, wherein said air-fuel ratiochangeover means changes over the air-fuel ratio with a time intervalfrom said predetermined lean air-fuel ratio to said predetermined richair-fuel ratio and from said predetermined rich air-fuel ratio to saidpredetermined lean air-fuel ratio, and said deterioration determiningmeans determines the degree of deterioration of the catalyst on thebasis of a mean value of said amount of oxygen calculated by saidcalculating means when the air-fuel ratio is changed over from saidpredetermined lean air-fuel ratio to said predetermined rich air-fuelratio and said amount of oxygen calculated by said calculating meanswhen the air-fuel ratio is changed over from said predetermined richair-fuel ratio to said predetermined lean air-fuel ratio.
 4. A deviceaccording to claim 1, wherein said calculating means calculates saidamount of oxygen OS on the basis of the following formula:

    OS=α·Δ(A/F)·Ga·ΔT

where α is a ratio of the amount of oxygen contained in air, Δ(A/F) issaid difference between the other predetermined air-fuel ratio and thestoichiometric air-fuel ratio, Ga is said amount of the exhaust gasdetected by said gas amount detecting means and ΔT is said time fromwhen air-fuel ratio is changed over from said predetermined richair-fuel ratio and said predetermined lean air-fuel ratio to the otherpredetermined air-fuel ratio to when the air-fuel ratio detected by saidair-fuel ratio sensor becomes approximately equal to the otherpredetermined air-fuel ratio.
 5. A device according to claim 4, whereinsaid gas amount detecting means detects an amount of air fed for burningthe fuel and uses said amount of the air as an amount representing theamount of the exhaust gas.
 6. A device according to claim 1, whereinsaid gas amount detecting means successively detects the amount of theexhaust gas passing through the catalyst per a given time during thetime from when the air-fuel ratio is changed over from one of saidpredetermined rich air-fuel ratio and said predetermined lean air-fuelratio to the other predetermined air-fuel ratio to when the air-fuelratio detected by said air-fuel ratio sensor becomes approximately equalto the other predetermined air-fuel ratio, and said calculating meanscalculates a total amount of oxygen stored in the catalyst based on asum of an amount of air calculated from said amount of the exhaust gasper said given time and a difference between the other predeterminedair-fuel ratio and the stoichiometric air-fuel ratio.
 7. A deviceaccording to claim 6, wherein said calculating means calculates saidtotal amount of oxygen OS on the basis of the following formulas:

    ΔOS=α·Δ(A/F)·Ga·Δt

    OS=[ΣΔS]ΣΔOS

where ΔOS is said amount of oxygen stored in the catalyst per said giventime, α is a ratio of the amount of oxygen contained in air, Δ(A/F) issaid difference between the other predetermined air-fuel ratio and thestoichiometric air-fuel ratio, Ga is said amount of the exhaust gasdetected by said gas amount detecting means, and Δt is said given time.8. A device according to claim 7, wherein said gas amount detectingmeans detects an amount of air fed for burning the fuel and uses saidamount of the air as an amount representing the amount of the exhaustgas.
 9. A device according to claim 1, wherein said air-fuel ratiochangeover means maintains the air-fuel ratio at one of saidpredetermined rich air-fuel ratio and said predetermined lean air-fuelratio for a predetermined time before changing over the air-fuel ratio,and said calculating means starts to calculate said amount of oxygenwhen said air-fuel ratio changeover means changes over the air-fuelratio.
 10. A device according to claim 1, wherein said device is used inan engine and said air-fuel ratio changeover means changes the air-fuelratio of an air-fuel mixture fed into the engine between saidpredetermined rich air-fuel ratio and said predetermined lean air-fuelratio.
 11. A device according to claim 10, further comprising memorymeans for storing said predetermined rich air-fuel ratio and saidpredetermined lean air-fuel ratio, said air-fuel ratio changeover meanscontrolling an amount of fuel fed into the engine to make the air-fuelratio of said air-fuel mixture equal to one of said predetermined richair-fuel ratio and said predetermined lean air-fuel ratio.
 12. A deviceaccording to claim 11, further comprising an additional air-fuel ratiosensor arranged in said exhaust gas passage upstream of the catalyst todetect an air-fuel ratio, said air-fuel ratio changeover meanscontrolling the amount of fuel fed into the engine on the basis of anoutput signal of said additional air-fuel ratio sensor to make theair-fuel ratio detected by said additional air-fuel ratio sensor equalto one of said predetermined rich air-fuel ratio and said predeterminedlean air-fuel ratio which are stored by said memory means.
 13. A deviceaccording to claim 12, in which the engine has a fuel injector forfeeding fuel into the engine, wherein a fuel injection time TAU by saidfuel injector is calculated based on the following formula:

    TAU=TP·FAF·GA·M

where TP is a basic fuel injection time determined by an operating stateof the engine, FAF is a feedback correction coefficient increasing anddecreasing relative to a reference value in accordance with a change inthe air-fuel ratio detected by said additional air-fuel ratio sensor tomake the air-fuel ratio of said air-fuel mixture equal to a targetair-fuel ratio, GA is a learning coefficient for maintaining the valueof FAF at said reference value, and M is an air-fuel ratio determiningcoefficient represented by the stoichiometric air-fuel ratio/said targetair-fuel ratio.
 14. A device according to claim 13, wherein saidair-fuel ratio changeover means makes the value of FAF said referencevalue and makes said target air-fuel ratio one of said predeterminedrich air-fuel ratio and said predetermined lean air-fuel ratio when theair-fuel ratio of said air-fuel mixture is equal to one of saidpredetermined rich air-fuel ratio and said predetermined lean air-fuelratio.